The work proposed is designed to understand dioxygen reduction by biological systems in molecular and mechanistic form. A heme sulfur protein, P450 cytochrome, carries the essential binding and catalytic sites on a single peptide of 45,000 daltons and a univalent ferric-ferrous redox couple to an iron sulfide protein (redoxin, Pd) of 12,000 daltons, with a selective micromolecular dissociation and effector role. The primary structure of Pd is known, and of the P450cam 70% completed. Analyses of tryptic and cyanogen bromine peptides identify 7 cystienyl, 3 Trp and 9 Met residues. The P450 tertiary structure determination is in progress by X-ray diffraction; effective complementation ensues from the concomitant primary and tertiary structure analyses. Essential residue modifications, by chemical and mutagenic reagents, are applied to identify invariant residues and functional aromatic and CysSH groups. The holo protein has been resolved and porphyrin exchanged with protein differentially labeled in stable isotopes of carbon, hydroen, oxygen, nitrogen and sulfur; cobalt has been substituted for iron in the prosthetic group. Resonance spectral structural identification by Mossbauer, nmr, epr, Endor, resonance Raman and optical probes (UV and visible) are proceeding. Dynamic and thermodynamic analyses partially completed are continuing over a 100 degrees K temperature range with variations in ionic strength K ion and H ion concentrations and solvent perturbations to identify regulatory and catalytic processes.